A magnetoresistive element having a ferromagnetic tunnel junction is also called a magnetic tunnel junction (MTJ) element. As a writing method for this element, a writing (spin transfer torque writing) method that uses spin-momentum-transfer (SMT) has been proposed.
It has been considered to use, as a ferromagnetic material that forms the magnetoresistive element, what is known as a perpendicular magnetization film having a magnetization easy axis (an axis of easy magnetization) in a direction perpendicular to a film plane. When magnetocrystalline anisotropy is used in a perpendicular magnetization configuration, shape anisotropy is not used, so that the element shape can be smaller than that of an in-plane magnetization configuration. Dispersion in a magnetization easy direction can also be reduced. Therefore, the use of a material having high crystalline magnetocrystalline anisotropy enables the maintenance of thermal disturbance resistance and also enables both miniaturization and current reduction.
A storage layer and a reference layer of the MTJ element include magnetic layers, and emit magnetic fields to the outside. In general, in an MTJ element having a storage layer and a reference layer that are a perpendicular magnetization type, a leakage magnetic field generated from the reference layer is greater than that of the in-plane magnetization MTJ element. The storage layer lower in coercive force than the reference layer is strongly influenced by the leakage magnetic field from the reference layer. More specifically, problems caused by the influence of the leakage magnetic field from the reference layer include the decrease of thermal stability, and the increase in the value of a switching current that switches the magnetization direction of the storage layer and in the variation of this switching current value.
A method of providing a shift adjustment layer to cancel the leakage magnetic field from the reference layer has been proposed as a measure for reducing the leakage magnetic field applied to the storage layer from the reference layer in the perpendicular magnetization MTJ element. When a material having saturation magnetization substantially equal to that of the reference layer is used for the shift adjustment layer which cancels the leakage magnetic field from the reference layer, it is necessary to design the shift adjustment layer which cancels the leakage magnetic field to be larger in thickness than the reference layer. Accordingly, the problem is that element fabrication becomes difficult as a result of the increased thickness of the MTJ element.